The use of precious metals to catalyze oxidation of carbon monoxide in the regenerators of FCC units has gained broad commercial acceptance. Some of the history of this development is set forth in U.S. Pat. No. 4,171,286 and U.S. Pat. No. 4,222,856. In the earlier stages of the development, the precious metal was deposited on the particles of cracking catalyst. Present practice is generally to supply a promoter in the form of solid particles containing a metal, such particles being physically separate from the particles of cracking catalyst. The precious metal, or compound thereof, is supported on particles of suitable carrier material and the promoter particles are usually introduced into the regenerator separately from the particles of cracking catalyst. The particles of promoter that are not removed from the system as fines are cocirculated with cracking catalyst particles during the cracking/stripping/regeneration cycles.
Promotor products that are presently used on a commercial basis in FCC units include calcined spray dried microspheres of kaolin clay impregnated with a small amount (e.g., 100 or 500 ppm) of platinum. Reference is made to U.S. Pat. No. 4,171,286 (supra) for its disclosure as to the production and properties of the calcined clay support material and the resulting promoter particles. Other commercially used promoters are obtained by impregnating a source of platinum on microspheres of high purity alumina. Commercially available sources of alumina-supported platinum were analyzed by the assignee of the instant patent application and found to analyze in the range of about 300 to 1000 ppm of platinum. The selection of platinum as the precious metal in the aforementioned commercial products appears to reflect a preference for this metal that is consistent with prior art showings that platinum is the most effective group VIII metal for carbon monoxide oxidation promotion in FCC regenerators. See, for example, FIG. 3 in U.S. Pat. No. 4,107,032 and the same figure in U.S. Pat. No. 4,350,614. The figure illustrates the effect of increasing the concentration of various species of precious metal promoters from 0.5 to 10 ppm on CO.sub.2 /CO ratio.
At least one promoter for FCC units that has been used commercially contains a precious metal (palladium) in addition to platinum. The support is gamma alumina. Our analysis of a sample of the product indicates that it contains 479 ppm Pd and 1154 ppm Pt (VF basis). To the best of our knowledge, the supplier of the product did not ascribe any particular benefit to the presence of palladium. In fact, a subsequent sample from the same supplier did not contain any palladium. A reasonable explanation for the presence of palladium with platinum is based on our understanding that such material was obtained by grinding waste material produced in the manufacture of oxidative automotive emission control catalysts. It is well known that catalysts for use in automotive emissions control generally include palladium because of its effectiveness in "light-off".
Another disclosure of the use of palladium in a mixed metal promoter of carbon monoxide oxidation in an FCC unit appears in U.S. Pat. No. 4,222,856. This patent describes promoters containing from about 500 ppm to about 1% by weight of group VIII metals on a solid support, preferably gamma-alumina. The patent discloses that the promoter preferably contains a mixture of platinum and palladium but no reason is advanced to explain the preference. U.S. Pat. No. 4,222,856 also teaches that a preferred proportion of platinum to palladium is from about 1.5-4.0:1 by weight and that the concentration of combined platinum and palladium is preferably from about 1,500 to 4,500 ppm. With respect to the support for the metal, there is an expressed preference for using a material of "friable" nature to permit removal of the promoter with the flue gas. In contrast, the calcined clay support of U.S. Pat. No. 4,171,286 and other commercially used promoters are attrition resistant and the bulk of the promoter particles circulate with catalyst inventory through the cracking, stripping and regenerator zones.
Recent patents disclose a preference for including palladium in promoters that are intended to oxidize oxides of sulfur in regenerator flue gases from FCC units. GB No. 2,104,406, for example, discloses the use of palladium in combination with platinum, osmium, iridium, rhenium, or rhodium to control sulfur oxides. Palladium purportedly helps to minimize the production of undesirable oxides of nitrogen in flue gas. As another example, U.S. Pat. No. 4,350,615 discloses the use of a palladium plus ruthenium based promoter for the control of emissions of nitrogen oxides and sulfur oxides. Neither G.B. No. 2,104,406 nor U.S. Pat. No. 4,350,615 discloses any advantage of palladium containing promoters for enhanced CO oxidation activity.
The present invention contemplates a novel combustion promoter in which the support is composed of attrition-resistant porous particles of caustic leached calcined clay, preferably particles of kaolin clay calcined to convert the kaolin into a mixture of mullite and free (leachable) silica, whereby the leached calcined clay consists essentially of aggregates of mullite rendered porous as a result of the extraction of free silica. The thermal conversion of kaolin clay to mullite is well known in the ceramics art and mullite is frequently mentioned as a support for various catalysts. A high purity kaolin clay can theoretically be converted by high temperature calcination into about 64% weight percent mullite. The remainder is an amorphous or crystalline silica, depending on the calcination conditions. The addition of various sources of alumina such as bauxite increases the amount of mullite that can be obtained from a given amount of kaolin, thereby reducing the amount of free silica. Similarly, mullite obtained from various other sources of silica and alumina, such as kyanite or bauxite, will not necessarily contain free silica.
The mullitization reaction that takes place when kaolin clay is utilized as the sole source of silica and alumina can be represented by the following equation wherein the approximate chemical formula for kaolin (without the water of hydration) is given as Al.sub.2 O.sub.3. 2SiO.sub.2, and the formula for mullite is 3Al.sub.2 O.sub.3.2SiO.sub.2 : EQU 3(Al.sub.2 O.sub.3.2SiO.sub.2).fwdarw.3Al.sub.2 O.sub.3.2SiO.sub.2 +4SiO.sub.2.
The term represented by 4SiO.sub.2 is the free silica generated as a result of the conversion to mullite.
Mullite is widely used in ceramic applications such as in the manufacture of refractory grains. For these applications, dense impervious products are needed and porosity is undesirable. See, for example, U.S. Pat. No. 3,642,505. It is known that a purified form of mullite can be obtained by calcining suitable clay such as kaolin, and extracting the silica with a strong base. See, for example, U.S. Pat. No. 2,536,122 and Japanese Patent Application No. 69 84,626 (CA81(10)53673a). It is our understanding that removal of the free silica in this fashion (or by reaction with sources of Al.sub.2 O.sub.3 to form additional mullite) is practiced to improve the refractoriness of the resultant solid. In this regard, it is noted that the Al.sub.2 O.sub.3 -SiO.sub.2 phase diagram (Phase Diagrams for Ceramists, Amer. Cer. Soc. Ed., 1964, Diagrams 313-314) shows that pure mullite does not melt until about 1850.degree. C.; however, in the presence of free silica, melting begins at only about 1600.degree. C. Therefore, by eliminating free silica, the refractoriness of mullite is improved to an extent such that the melting point is about 250.degree. C. higher.
Copending U.S. application, Ser. No. 505,650, filed June 20, 1983 in the name of Barry K. Speronello, describes novel particulate aggregates in the form of shaped bodies, such as microspheres, and comprising mullite crystals as the sole or predominating solid component, the products being characterized by relatively high surface area; e.g., greater than about 15 m.sup.2 /g, high pore volume, e.g., greater than about 0.22 cc/g, and a high concentration of pores in the range of 150 to 350A (Angstrom) diameter. The aggregates are prepared by leaching silica from preformed shaped bodies of clay calcined to form mullite and free silica by means of a caustic solution, preferably sodium hydroxide. Representative products described in Ser. No. 505,650 analyze at least about 50% mullite, as determined by X-ray analysis, and preferably at least 75% mullite, and most preferably at least 85% mullite. This corresponds to a molar ratio of Al.sub.2 O.sub.3 /SiO.sub.2 that is in excess of 0.5, usually in excess of 1.00 and up to about 1.65. The Al.sub.2 O.sub.3 /SiO.sub.2 molar ratio of high purity kaolin clay is about 0.5.
The aforementioned patent application further describes preferred uses for the porous mullite products as being those taking advantage of their unique combination of high pore volume (particularly mesoporosity), controllable surface area (from moderate to high, e.g., from 15 to 60 m.sup.2 /g), excellent refractoriness and high strength. Disclosures of preferred uses include supports for catalysts used to oxidize carbon monoxide or hydrocarbons. Also disclosed in said patent application is that microspherodial porous mullite articles having a surface area between about 20 m.sup.2 /g and 50 m.sup.2 /g and hardness at least comparable to that of commercially useful fluid cracking catalysts may be useful as a support for a precious metal for the oxidation of carbon monoxide in the regenerator of a FCC unit. The application further disclosed that the CO oxidation promoter can be made by impregnating porous mullite microspheres to incipient wetness with a chloroplatinic acid solution of appropriate concentration to result in from about 100 to 500 ppm of Pt being deposited on the support.